1. Field of the Invention
The present invention relates to methods for recovering phosphate from sludge when sewage is treated using activated sludge at sewage treatment plants, in which, by treating waste sludge or mixed sludge of waste sludge and sludge from primary settling basins in an anaerobic condition, phosphate contained in the waste sludge are released into solution, and phosphate in the solution are recovered. The invention also relates to systems for recovering phosphate from sludge.
2. Description of the Related Art
In order to control water pollution in rivers, lakes, and enclosed coastal sea areas, sewage treatment plants for treating industrial and municipal sewage have been built in many places.
FIG. 9 shows an example of a system for treating sewage and sludge conventionally adopted in such sewage treatment plants.
With respect to sewage received at a plant, first, sand, coarse refuse, etc., are removed at a sand separation basin.
Next, suspended solids are settled in a primary settling basin, and supernatant is obtained. The supernatant is treated in a biological reaction tank in an aerobic condition, using activated sludge. Organic substances in the supernatant are primarily removed thereby.
Next, in a secondary settling basin, mixed liquor suspended solids are separated into activated sludge and treated water. The activated sludge is fed to a subsequent stage, and the separated treated water is discharged from the plant as effluent.
The separated activated sludge is separated into return sludge and waste sludge. The return sludge is returned to the biological reaction tank and is reused as activated sludge. The waste sludge is treated in a sludge treatment system.
The waste sludge generated in the water treatment system and sludge from the primary settling basin (raw sludge), which is settled in the primary settling basin, are thickened individually or as a mixture of both in the sludge treatment system, and separation between thick sludge and liquid separated by thickening is achieved. The liquid separated by thickening is returned to the water treatment system. The separated thick sludge is subjected to dewatering treatment and is separated into liquid separated by dewatering and dewatered sludge. The liquid separated by dewatering is returned to the water treatment system as sidestreams. The separated dewatered sludge is disposed of in landfills, by incineration, or other appropriate methods.
However, in the conventional treatment method described above, the following problems may occur. That is, the treated water contains substances, such as phosphorus and nitrogen, which are responsible for eutrophication. Therefore, the treated water is a contributing factor to red tide and the like in lakes, enclosed coastal sea areas, etc.
As a countermeasure against the above, in the water treatment system described above, an anaerobic tank may be provided before the aerobic biological reaction tank. Thereby, activated sludge accumulates phosphate (orthophosphate ions) as polyphosphate, and the phosphorus concentration in the effluent is reduced.
After the activated sludge which has accumulated polyphosphoric acid is withdrawn as waste sludge into the sludge treatment system, phosphate may be released from the waste sludge depending on a thickening method, resulting in increases in the phosphate concentration in the liquid separated by thickening and liquid separated by dewatering.
The liquid separated by thickening and liquid separated by dewatering having increased phosphate concentrations are returned to the water treatment system as sidestreams. As a result, effects of reducing the phosphorus concentration in the effluent are reduced.
An anaerobic digestion tank may be provided subsequently to a thickener. In such a case, when digested sludge which has been digested at the anaerobic digestion tank is transported to a dewatering apparatus, reactions take place among phosphorus, magnesium ion, ammonia, etc., released into the solution by anaerobic digestion, and magnesium ammonium phosphate crystals, i.e., struvite are precipitated in pipes and pumps. As the precipitation progresses, accidents such as clogging of the sludge transport facility may occur.
In order to overcome the problems described above, currently, coagulating agents, such as lime, aluminum sulfate, ferric sulfate, and polyaluminum chloride, are used in sludge treatment processes. When coagulating agents are used, the amount of sludge is increased due to the added coagulant. Consequently, final disposal expenses for sludge associated with sewage treatment are increased. Since products formed by reactions between the coagulant and phosphate are chemically stable, they are not reusable for fertilizers. The products are contained in the dewatered sludge at the end. Therefore, when dewatered sludge is used as a raw material for cement, which has been receiving attention as a final disposal technique of sewage sludge, the cement is degraded by the phosphate. As a result, it is difficult to adopt such a final disposal technique.
It is an object of the present invention to provide a method and a system for efficiently recovering phosphate from mixed sludge.
It is another object of the present invention to provide a method and a system for recovering phosphate in a form which can be effectively used as a fertilizer.
It is another object of the present invention to provide a method and a system for recovering phosphate, in which the highly efficient recovery of phosphate is simplified.
It is another object of the present invention to provide a method and a system for recovering phosphate, in which clogging of sludge transport pipes is avoided.
It is another object of the present invention to provide a method and a system for recovering phosphate, in which phosphate are recovered from sludge with high efficiency so as to prevent treated water from affecting the environment.
According to a first aspect of the present invention, a method for recovering phosphate from sludge includes the processes of treating sludge discharged from a water treatment system at a sewage treatment plant in an anaerobic condition to release phosphate in the sludge into solution, and recovering phosphate in the solution using a seed crystal material.
The sludge includes, for example, waste sludge generated in the water treatment system, and mixed sludge of waste sludge and raw sludge.
The treatment in an anaerobic condition is performed, for example, by injecting sludge into an anaerobic tank and retaining it in the anaerobic tank for a predetermined period of time. In the treatment in an anaerobic condition, phosphate (polyphosphate) in waste sludge (solids) are subjected to hydrolysis and are released as orthophosphate into sludge solution.
Phosphate released into the solution are recovered, using a seed crystal material, for example, as hydroxyapatite crystals on the surface thereof.
As the seed crystal material, for example, a substantially spherical material containing a calcium silicate hydrate as a major constituent is used. The calcium silicate hydrate may be at least one selected from the group consisting of tobermorite, xonotlite, hillebrandite, and wollastonite. In addition to the above, a seed crystal material disclosed in Japanese Unexamined Patent Publication No. 9-308877 may be used. In this material, a calcium silicate hydrate is attached to the surface of a honeycomb core. As a seed crystal material, autoclaved lightweight concrete ALC, may also be used. In the production of ALC, a calcic material and a silicic material are used as major materials, and after water and a foaming agent are added thereto, autoclave curing is performed.
The treatment in an anaerobic condition and the recovery of phosphate may be performed separately or in the same tank.
According to a second aspect of the present invention, a method for recovering phosphate from sludge according to the first aspect of the invention further includes the process of digesting the sludge treated in an anaerobic condition in an anaerobic digestion tank, and dewatering and separating the digested sludge into dewatered sludge and liquid separated by dewatering.
After the sludge having a decreased phosphorus content is treated in the anaerobic digestion tank, dewatering is performed to achieve the separation between the dewatered sludge and the dewatering effluent. At this stage, since the phosphorus content of the digested sludge treated in the anaerobic digestion tank is preliminarily decreased, even if phosphate are released from solids of the digested sludge, the possibility of precipitation of struvite in pipes and pumps is significantly reduced.
According to a third aspect of the present invention, in a method for recovering phosphate from sludge according to one of the first and second aspects of the invention, the process of recovering phosphate using a seed crystal material includes a process of adjusting the calcium ion concentration in the solution to a range from 80 to 150 mg/l.
If the calcium ion concentration for adjusting calcium ions in the solution to be treated is less than 80 mg/l, the calcium ion concentration is insufficient, resulting in a decrease in the phosphate recovering efficiency. Even if the concentration is set at more than 150 mg/l, the efficiency is not increased, which is uneconomical. Products resulting from precipitation of calcium ions are also generated, and more sludge requiring disposal is produced.
According to a fourth aspect of the present invention, in a method for recovering phosphate from sludge according to any one of the first to third aspects of the invention, the process of recovering phosphate using a seed crystal material includes a process of adjusting the pH of the solution to a range from 7.5 to 9.0.
If the pH is less than 7.5, hydroxyapatite is not generated. If the pH exceeds 9.0, products resulting from precipitation of calcium ions are generated and hydroxyapatite microcrystals are generated in water, which are discharged from the phosphate recovering tank, resulting in a decrease in the phosphate recovery efficiency. There is also a possibility that products resulting from precipitation of calcium are generated and inorganic sludge is produced.
According to a fifth aspect of the present invention, in a method for recovering phosphate from sludge according to any one of first to fourth aspects of the invention, the treatment in an anaerobic condition is performed simultaneously with the recovery of phosphate using a seed crystal material.
According to a sixth aspect of the present invention, in a method for recovering phosphate from sludge according to the fifth aspect of the invention, the seed crystal material flows in the sludge solution.
According to a seventh aspect of the present invention, in a method for recovering phosphate from sludge according to the sixth aspect of the invention, the seed crystal material contains a calcium silicate hydrate as a major constituent and is formed in a substantially spherical shape.
According to an eighth aspect of the present invention, a method for recovering phosphate from sludge includes the process of treating sludge discharged from a water treatment system at a sewage treatment plant in an anaerobic condition to release phosphate in the sludge into solution, separating the sludge treated in an anaerobic condition into thickened sludge and thickener effluent, dewatering and separating the thickened sludge into dewatered sludge and dewatering effluent, and recovering phosphate contained in the dewatering effluent and the thickener effluent.
Thickening of the sludge may be performed by any method such as gravity thickening, floatation thickening, and centrifugal thickening. The water content in the sludge can be decreased by thickening. Thus, the phosphorus content in the thickened sludge can be decreased.
Separation by dewatering may also be performed by any method.
In this way, since phosphate are recovered both from the thickener effluent and from the dewatering effluent, the recovery rate of phosphate from the sludge can be increased.
With respect to the recovery of phosphate, in addition to the method using seed crystal material described above, rock phosphate, bone black, silica, calcite and ALC may be employed as seeding materials.
According to a ninth aspect of the present invention, a method for recovering phosphate from sludge according to the eighth aspect of the invention further includes the process of digesting the thickened sludge at an anaerobic digestion tank, and dewatering and separating the digested sludge into dewatered sludge and dewatering effluent.
After the sludge having a decreased phosphorus content is treated in the anaerobic digestion tank, dewatering is performed to achieve separation between the dewatered sludge and dewatering effluent. At this stage, since the phosphorus content of the digested sludge treated in the anaerobic digestion tank is preliminarily decreased, even if phosphorus components are released from solids of the digested sludge, the possibility of precipitation of in the pipes and pumps is significantly reduced.
According to a tenth aspect of the present invention, in a method for recovering phosphorus components from sludge according to one of the eighth and ninth aspects of the invention, the process of recovering phosphates includes a process of adjusting the calcium ion concentration in the solution to a range from 80 to 150 mg/l.
The calcium ion concentration for adjusting calcium ions in sludge slurry is preferably set at 80 to 150 mg/l. If the calcium ion concentration is less than 80 mg/l, the calcium ion concentration may be insufficient depending on the characteristics of the sludge to be treated, resulting in a decrease in the phosphate recovering efficiency. Even if the concentration is set at more than 150 mg/l, no effect is obtained and inorganic sludge is increased.
According to an eleventh aspect of the present invention, in a method for recovering phosphate from sludge according to any one of the eighth to tenth aspects of the invention, the process of recovering phosphate includes a process of adjusting the pH of the solution to a range from 7.5 to 9.0.
If the pH is less than 7.5, hydroxyapatite is not generated. If the pH exceeds 9.0, products resulting from precipitation of calcium ions are generated and hydroxyapatite microcrystals are generated in water, which are discharged out of a phosphate recovery tank, resulting in a decrease in the phosphate recovery efficiency. There is also a possibility that products resulting from precipitation of calcium are generated and inorganic sludge is produced.
According to a twelfth aspect of the present invention, in a method for recovering phosphate from sludge according to any one of the eighth to eleventh aspects of the invention, in the process of recovering phosphate, a seed crystal material is used.
As the seed crystal material, as described above, various types of materials may be used.
According to a thirteenth aspect of the present invention, in a method for recovering phosphate from sludge according to the twelfth aspect of the invention, the seed crystal material contains a calcium silicate hydrate as a major constituent and is formed in a substantially spherical shape.
According to a fourteenth aspect of the present invention, a system for recovering phosphate from sludge includes a phosphorus-releasing means for treating waste sludge drawn from a water treatment system at a sewage treatment plant in an anaerobic condition to release polyphosphate accumulated in sludge into bulk liquid, a dewatering and separating means for separating the sludge mixed liquor containing the solution including the released phosphate into dewatering effluent and dewatered sludge, a calcium ion concentration-adjusting means for adjusting the calcium ion concentration in the dewatering effluent, a pH-adjusting means for adjusting the pH of the dewatering effluent, and a means for recovering phosphate from the calcium ion concentration-adjusted, pH-adjusted effluent of dewatering process.
According to a fifteenth aspect of the present invention, a system for recovering phosphate from sludge includes a phosphoms-releasing means for treating waste sludge drawn from a water treatment system at a sewage treatment plant in an anaerobic condition to release polyphosphate accumulated in sludge into bulk liquid, a thickening means for thickening and separating the sludge mixed liquor containing the solution including the released phosphate into thickener effluent and thickened sludge, a dewatering and separating means for separating the thickened sludge into dewatering effluent and dewatered sludge, a calcium ion concentration-adjusting means for adding a water soluble calcium compound to the thickener effluent and the dewatering effluent to adjust the calcium ion concentration therein, a pH-adjusting means for adding an alkaline substance to the thickener effluent and the dewatering effluent to adjust the pH thereof, and a means for recovering phosphate from the calcium ion concentration-adjusted, pH-adjusted effluents of thickener and dewatering process.
According to a sixteenth aspect of the present invention, a system for recovering phosphate from sludge according to the fifteenth aspect of the invention further includes an anaerobic digesting means for digesting the thickened sludge.
According to a seventeenth aspect of the present invention, in a system for recovering phosphate from sludge according to any one of the fourteenth to sixteenth aspects of the invention, the means includes a seed crystal material for recovering phosphate by crystallizing reaction.
According to an eighteenth aspect of the present invention, a system for recovering phosphate from waste sludge includes a recovery tank for receiving waste sludge drawn from a water treatment system at a sewage treatment plant, a phosphorus-releasing means for treating the waste sludge in an anaerobic condition in the recovery tank to release polyphosphate into bulk liquid, a calcium ion concentration-adjusting means for adjusting the calcium ion concentration in the liquid containing phosphate, a pH-adjusting means for adjusting the pH of the liquid, and a means for recovering phosphate from the liquid.
Accordingly, an apparatus for recovering phosphate is simplified.
The releasing and recovery of phosphate in waste sludge or mixed sludge can be performed in the same tank, whereby, a thickening tank can be omitted.
According to a nineteenth aspect of the present invention, in a system for recovering phosphate from sludge according to the eighteenth aspect of the invention, the means includes a seed crystal material for recovering phosphate by crystallizing reaction.
According to a twentieth aspect of the present invention, in a system for recovering phosphate from sludge according to the nineteenth aspect of the invention, the seed crystal material comes into contact with the sludge in a fluid state for crystallization.
According to a twenty-first aspect of the present invention, a system for recovering phosphate from sludge according to any one of the eighteenth to twentieth aspects of the invention further includes a thickening means for thickening and separating the sludge dephosphorized by the dephosphorizing means into thickener effluent and thickened sludge, and an anaerobic digesting means for digesting the thickened sludge.
Accordingly, the possibility of precipitation of struvite in the transport pipes and pumps for digested sludge is reduced, thus preventing clogging of the sludge transport facility. Consequently, stable operations can be secured, and treatment efficiency for the entire sludge treatment can be improved.
According to a twenty-second aspect of the present invention, in a system for recovering phosphate from sludge according to any one of the seventeenth, and nineteenth to twenty-first aspects of the invention, the seed crystal material contains a calcium silicate hydrate as a major constituent and is formed in a substantially spherical shape.
According to a twenty-third aspect of the present invention, in a system for recovering phosphate from sludge according to the twenty-second aspect of the invention, the calcium silicate hydrate is at least one selected from the group consisting of tobermorite, xonotlite, hillebrandite, and wollastonite.
According to a twenty-fourth aspect of the present invention, in a system for recovering phosphate from sludge according to any one of the fourteenth to twenty-third aspects of the invention, the calcium ion concentration-adjusting means adjusts the calcium ion concentration to a range from 80 to 150 mg/l.
The calcium ion concentration for adjusting calcium ions in the raw water to be treated is preferably set at 80 to 150 mg/l. If the calcium ion concentration is less than 80 mg/l, the calcium ion concentration is insufficient, resulting in a decrease in the phosphate recovering efficiency. Even if the concentration is set at more than 150 mg/l, the efficiency is not increased, which is uneconomical. Products resulting from precipitation of calcium ions are also generated, and sludge to be disposed of is thereby increased.
According to a twenty-fifth aspect of the present invention, in a system for recovering phosphate from sludge according to any one of the fourteenth to twenty-fourth aspects of the invention, the pH-adjusting means adjusts the pH to a range from 7.5 to 9.0.
If the pH is less than 7.5, hydroxyapatite is not generated. If the pH exceeds 9.0, products resulting from precipitation of calcium ions are generated and hydroxyapatite microcrystals are generated in bulk liquid, which are wasted out from a phosphate recovery tank, resulting in a decrease in the phosphate recovery efficiency. There is also a possibility that products resulting from precipitation of calcium are generated and inorganic sludge is produced.
Additionally, by using calcium hydroxide, the calcium ion concentration and the pH may be simultaneously adjusted. In such a case, while crystallizing reaction of hydroxyapatite is taking place, the pH is not greatly increased, and when calcium ions are excessively increased, the pH increases. Thus, by controlling the pH within a predetermined range, calcium ions are prevented from being excessively added. Therefore, the simplification of the apparatus and a reduction in the chemical cost by use of calcium hydroxide can be achieved.
By adjusting the calcium ion amount to be added to the thickener effluent and the dewatering effluent as well as the pH, the seed crystal material can be used with high efficiency for a long period of time.
Furthermore, used seed crystal material can be reused as silicic fertilizers, phosphate fertilizers. The used seed crystal material can also be used for phosphate industry.
Since the dewatered sludge treated with the system in accordance with the present invention has a low phosphorus component content, when it is used as a raw material for cement, the degradation of cement by harmful phosphorus components can be avoided.